1. Field of the Invention
The present invention relates to a density level compensation for thermal disturbance for stably reproducing density in a printing apparatus for a multi-gradation image of high definition such as a television screen of the NTSC system, a computer graphics (CG), or a high definition television by using a thermal head.
2. Description of the Related Art
Recently, a thermal printing method for performing thermal printing by using a thermosensible printing paper or a thermal transfer film is superior to an ink-jet method and an electrophotographic method, because color printing can be easily realized and the apparatus size can be minimized by the thermal printing method. Moreover, the thermal printing method is advantageous in the image quality, the cost, and the maintenance of the apparatus. For the above reasons, the thermal printing method is widely applied to a hard copy apparatus for printing a photograph-like image.
In general, in a color printer utilizing a thermal gradation printing method, a line thermal head in which heat-elements are arranged in a line, and an ink sheet which is divisionally colored in yellow (Y), magenta (M) and cyan (C) are used. For one color, the printing is performed in a line sequence. When the printing for one color is completed, the image receiving sheet is rewound and the printing for the next color is performed. In this way, the printings for three colors are performed in a face sequence. In order to print a photograph-like image, a sublimation dye thermal transfer printing method and a concentrated heating transfer printing method are superior both of which can maintain sufficient resolution and gray scale, can easily control printing density, and can perform smooth gradation printing.
However, both of the methods utilize a heating energy generated by energizing heat-elements in the thermal head, so that the printed density is influenced by thermal disturbances, such as ambient temperature variation and heat accumulations in the thermal head. As a result, it is difficult to always stably reproduce density. In order to stabilize the printed density, the control for driving the thermal head that considers the temperature dependency is performed. Such a control is referred to as a density level compensation for thermal disturbance. The density level compensation for thermal disturbance is a main factor which limits the improvement in image quality during the development of such a printer.
When the full color printing by the face sequence is considered, the density balance between colors are broken due to different ambient temperatures or different accumulated heat amounts for respective colors. This results in the change of chromaticity of the printed color, so that strict requirements are required for the density level compensation for thermal disturbance.
For solving the above problem, there has been proposed a gradation printer (U.S. Pat. No. 5,066,961) as a first conventional example. In such a printer, an average accumulated heat amount in the substrate in the thermal head is estimated, and the time period for supplying power to the thermal head is corrected in accordance with the temperature variation due to the heat accumulation of the thermal head, by using the temperature of the body portion in the thermal head and the average accumulated heat amount in the substrate. As a result, the density is stably reproduced.
As a second conventional example, a thermal printing apparatus (Japanese Laid-Open Patent Publication No. 2-248264) has been proposed. In this apparatus, the thermal resistance and the thermal time constant which determines the thermal history in the substrate of the thermal head are automatically set, and the temperatures of regions of the substrate corresponding to respective heat elements in the thermal head are estimated. Thus, the time period for supplying power to the thermal head is corrected, so that the density is stably reproduced.
As a third and a fourth examples, a heat accumulation correcting circuit for a thermal head (Japanese Laid-Open Patent Publication No. 2-289364) and a heat accumulation estimating circuit (Japanese Laid-Open Patent Publication No. 3-24972) have been proposed. In such circuits, the accumulated heat amount along the main-scanning direction in the thermal head is obtained for each heat element for each 1-line printing period.
In a thermal head of a thin film type which is generally used, there exist three types of heat accumulations, i.e., a first heat accumulation in the body portion mainly caused by the thermal capacitance of the body portion and the heat dissipation to the air, a second heat accumulation in the substrate, and a third heat accumulation in a heat element, which respectively. have time constants largely different from each other by about several minutes, several seconds or several milliseconds.
For the density level compensation for thermal disturbance in the gradation printing, it is required that the density correction accuracy be improved to a level corresponding to the gradation steps, so that the density of each gradation step can be accurately reproduced at any ambient temperature.
A thermal transfer printer or the like which is currently called a video printer makes a hard copy of an NTSC video image having a relative small image size to be printed (e.g., the A6 size). In such a printer, most of the input images are natural images having relatively averaged density distribution, and the line thermal head is short. Accordingly, such a printer has little degradation in the image quality due to the influence of the heat accumulation in the main-scanning direction. For this reason, as in the first conventional example, the density level compensation for thermal disturbance is performed based on the variation in ambient temperature, and an averaged accumulated heat amount in the main-scanning direction in the thermal head.
However, when an image with a greatly higher resolution than the NTSC video image, such as a high definition video image is to be printed, the gradation reproducibility and color reproducibility with higher accuracy are required. In the printing of an image having drastic density changes along the main-scanning direction (for example, an image having drastic density changes along the main-scanning direction as well as the sub-scanning direction, such as a computer graphic image), the accumulated heat amount is not uniform along a longitudinal direction of the thermal head (i.e., along the main-scanning direction of the image). As a result, the influence by such non-uniform accumulated heat amount may reach a level which cannot be negligible.
Moreover, with the development in office automation, it is essential to use a thermal head capable of printing an image having the size of A4 or more. Such a thermal head is longer than a thermal head used in a video printer. For the density level compensation for thermal disturbance in the first conventional example, the accumulated heat amount in the substrate is represented by an average value along the main-Scanning direction. In the second conventional example, the heat accumulation An the substrate and the cooling of the substrate are considered for each heat element. However, the second conventional example does not consider the heat accumulation and the cooling along the longitudinal direction of the thermal head such as the heat inflow and diffusion caused by the heat generation by the adjacent heat elements. Therefore, when an image having drastic density changes along the main-scanning direction is to be printed by a hard copy for a larger image with higher quality, the variation in printed density cannot be sufficiently corrected by the first and second conventional examples. In some cases, there may occur overcompensation which deteriorates the image quality.
The third and fourth conventional examples have the following problems. It is difficult to actually measure the accumulated heat amount for each heat element in the main-scanning direction, and a method for correcting an applied energy by using the accumulated heat amount in the main-scanning direction is not established. Therefore, the correction of the applied energy is performed by using the correction value which is determined on the basis of a lot of data obtained by experiments, simulations, etc. However, the correction value thus determined can be used only under the corresponding printing conditions. Therefore, a correction value for the other printing conditions should be determined based on experiences or trials. Thus, it is extremely difficult to accurately reproduce the density of all gradation levels in the third and fourth conventional examples.
Furthermore, none of the above conventional examples, the third heat accumulation in heat elements which have a relatively little influence on the printed density during the low-speed printing is not considered. Therefore, the conventional examples have a problem in that there may occur the deterioration in image quality such as dullness of image edges due to the third heat accumulation in heat elements when the high-speed and high-quality printing is to be performed.
In a gradation printer capable of printing a full-color image, at least 64 gradation levels (6 bits) are required. In most conventional cases, the number of gradation levels is 256 (8 bits), because the 8-bit data is mainly used as the input digital RGB data, and because human beings can recognize an image to be full-color if 256 gradation levels are provided for each color. Accordingly, it is necessary to set the pulse width data for setting a time period supplying a power to the thermal head to be at least 8-bit data. If the pulse width data for setting a time period supplying the power to the thermal head is limited to 8-bit data, it is impossible to realize the correction accuracy higher than 1/256 determined by the 8-bit data. By increasing the number of bits of the corrected pulse width data and the correcting coefficient from 8 bits, it is possible to improve the correction accuracy. However, it is necessary to drive the thermal head in accordance with the pulse width data for the same time period, even if the number of bits is increased. Accordingly, for every increase by one bit in the data transfer to the thermal head, substantially the double processing speed is required. Such a higher processing speed results in a larger increase in the circuit scale, or the lake. As described above, the correction accuracy is in conflict with the circuit scale necessary for the data transfer to the thermal head.